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FM recording on Audio Cassettes
In , on 01/03/04
at 07:27 PM, (Curious) said:
Hi:
Why do cassettes use FM recording?
Why not AM recording?
How would the artifacts differ?
Common audio tape recording does not use AM or FM. In its original
concept, the audio signal is run through a coil that is pressed against
the moving tape. Since the coil current varies with the audio signal,
an magnetic impression of the audio signal is "recorded" onto the tape.
During playback, the moving tape is again pressed against the coil,
current is generated in the coil as the varying magnetic field passes
by, this current is amplified and eventually drives the speakers.
Unfortunately, the above simplistic process does not produce the low
distortion audio that we need. A high frequency (compared to the audio
signal) steady current is added to the audio current presented to the
recording head (coil). This "bias" signal reduces distortion to a
manageable level.
The physical process of tape bias is very different from the AM and FM
modulation used for broadcasting.
It is possible to introduce an AM or FM modulation layer to the tape
recording process, and this is done for certain industrial
applications, but these processes are not used for home audio tapes.
---
AC bias was discovered independently in Germany and the US during the
1920's, but deployment was interrupted by political events. (I don't
know who was first. I've seen claims both ways.)
During the Second World War the German generals' would broadcast
speeches. Early in the war these broadcasts would be live or
transcribed to records because there was no practical way to send high
quality audio over phone wires and microwave relay links were not yet
available. Recordings and broacasts had to originate in a studio
setting. Therefore, the allies could easily track the location of a
general by monitoring the quality of his broadcast voice. While many
transmitters might carry the speech, one had much higher quality and
exposed the live location of the general. Eventually the Germans
started using AC bias magnetic recordings that sounded good enough to
prevent the allies from tracking the location of generals.
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